Tay-Sach's Disease

1. DEFINITION AND CLASSIFICATION

• Definition: A fatal, progressive neurodegenerative Lysosomal Storage Disorder (LSD) caused by deficiency of the enzyme Beta-Hexosaminidase A.
• Classification:
  • Infantile (Classic): Onset <6 months, rapid progression, death by 4-5 years.
  • Juvenile: Onset 2-10 years, ataxia, progressive cognitive decline.
  • Adult (Late-Onset): Onset in adolescence/adulthood, motor neuron disease-like presentation, psychosis.

2. ETIOLOGY AND GENETICS

• Inheritance: Autosomal Recessive.
• Gene: HEXA gene on chromosome 15q23.
• Enzyme Defect: Deficiency of Beta-Hexosaminidase A (specifically the alpha-subunit).
  • Note: Deficiency of Beta-Hexosaminidase A & B occurs in Sandhoff Disease (beta-subunit defect).
• Epidemiology: High carrier frequency (1 in 27) in Ashkenazi Jewish population. Also increased in French Canadians and Cajuns.
• Pathophysiology:
  • Inability to degrade GM2 ganglioside.
  • Accumulation of GM2 ganglioside in neuronal lysosomes causes ballooning of neurons, cellular dysfunction, and apoptosis.

3. CLINICAL FEATURES (CLASSIC INFANTILE FORM)

• Onset: Infants appear normal until 3-5 months of age.
• Early Signs:
  • Hyperacusis: Exaggerated startle response to sharp sounds (often the earliest and most characteristic sign).
  • Poor visual fixation/tracking.
  • Hypotonia and listlessness ("floppy infant").
  • Loss of previously acquired motor milestones.
• Ophthalmologic:
  • Cherry-Red Spot: Present in >90% of cases. Caused by lipid-laden whitish ganglion cells surrounding the normal red fovea centralis.
  • Optic atrophy and eventual blindness.
• Neurologic Progression:
  • Macrocephaly: Develops after 12-18 months due to reactive cerebral gliosis (brain overgrowth), not hydrocephalus.
  • Spasticity, hyperreflexia, and extensor plantar responses replace initial hypotonia.
  • Seizures: Tonic-clonic or myoclonic, often refractory to treatment.
  • Decerebrate posturing.
• Systemic: NO Hepatosplenomegaly (Important distinction from Niemann-Pick and Sandhoff Disease).

4. INVESTIGATIONS

• Enzyme Assay (Gold Standard):
  • Demonstration of absent or markedly reduced Beta-Hexosaminidase A activity in serum, leukocytes, or fibroblasts.
  • Total hexosaminidase activity may be normal or elevated (due to compensatory increase in Hex B).
• Molecular Genetics: HEXA gene sequencing (confirms diagnosis and identifies specific mutations).
• Imaging (MRI Brain):
  • Thalamic hyperdensity on CT (suggestive early sign).
  • T2 hyperintensity in basal ganglia/white matter.
  • Progressive cerebral atrophy in late stages.

5. MANAGEMENT

• Curative: None available currently.
• Supportive Care:
  • Nutrition: Nasogastric or gastrostomy tube feeding for dysphagia/aspiration risk.
  • Seizure Control: Antiepileptics (often difficult to control).
  • Respiratory: Suctioning, chest physiotherapy for secretion management.
• Prevention:
  • Carrier Screening: Enzyme assay or DNA testing recommended for high-risk populations (Ashkenazi Jews).
  • Prenatal Diagnosis: CVS or amniocentesis for enzyme analysis.

6. PROGNOSIS

• Infantile Form: Invariably fatal. Death typically occurs by 2 to 4 years of age, usually due to bronchopneumonia.